The White River Badlands contain the largest assemblage of known late Eocene and Oligocene mammal fossils. Fossil research from the area contributed significantly to the science of vertebrate paleontology in North America, beginning with the description of a titanothere mandible in 1846 by Dr. Hiram Prout. Since then numerous important finds from the area have informed scientists about ancient animals, climates, and ecosystems from different geologic time periods. Oligocene fossil remains include camels, three-toed horses, oreodonts, antelope-like animals, rhinoceroses, deer-like mammals, rabbits, beavers, creodonts, land turtles, rodents and birds.
Marine fossils are found in deposits of an ancient sea that existed in the region some 75 to 67 millionyears ago during the Cretaceous period. Fossils found in the Pierre Shale and Fox Hills Formations include ammonites, nautiloids, fish, marine reptiles, and turtles.
The spectacular vertebrate fossils preserved within the White River Badlands have been studied extensively since 1846 and are included in museum collections throughout the world.